Recent studies have shown that heme is involved in regulation of the formation and/or assembly of several cytochromes. Heme deprivation resulted in partial or total loss of cytochrome C1, as well as of subunits V and VII of cytochrome oxidase from yeast mitochondria. The goal of this project is to clarify the mechanism by which this relatively small cofactor exerts such control. The effect of heme on transcription of the genes for apocytochrome c1 and for subunits V and VII of cytochrome oxidase will be studied using messenger RNA from heme-deficient yeast strains in an in-vitro translation system. Polypeptide products will be isolated, identified and quantified by immunoprecipitation with subunit-specific antisera followed by gel electrophoresis. If the pertinent products are found in normal amounts under conditions of heme deficiency, a role for heme in transcription can be ruled out. In-vivo experiments involving pulse-labelling and pulse-chase will be performed to determine if heme is involved in either accumulation of the polypeptides or in the assembly of the holoenzyme in the mitochondrial membrane. Since the apoprotein moieties involved have been shown in wild-type cells to be translated as precursor polypeptides, experiments to assess the role of heme in the maturation process will be performed. If the translation experiments show that under heme deficient conditions, the pertinent protein products are missing or decreased then a role for heme in transcription or post-transcriptive processing would be indicated. The involvement of heme in essential biologic activities of mammalian cells, makes it important to understand the role of this molecule in regulation protein synthesis as well as in processes involving assembly and integration of macromolecular structures.